1. Technical Field
The invention relates to an improved engine capable of minimizing emissions.
2. Description of Related Art
Internal combustion engine designers continue to confront an ever more demanding set of governmentally mandated emissions standards and performance objectives. Modifications made to meet one standard may lead to increased emissions of a type that cause another standard to be exceeded. Thus designers are often confronted with not only the challenge of meeting a newly imposed emission standard but to do so in a way that does not cause other emissions standards, previously met or newly imposed, to be exceeded. The engine designers must also necessarily consider and preferably minimize the adverse effects of modifications on engine performance and fuel economy.
An example of the difficulties created for engine designers is that created by a new set of diesel engine emissions standards/limits mandated by the Environmental Protection Agency for application in the U.S. market. These standards require diesel engines to produce extremely low levels of emissions below specific limits based upon fuel consumption. Specifically, for example, new on-highway regulations require diesel engines complying with the regulations to maintain nitrogen oxide (NOx) emissions combined with unburned hydrocarbons below 2.5 grams/b-hp-hr and particulates below 0.1 grams/b-hp-hr.
Changes in any one of a variety of engine design variables or engine operating variables such as engine compression; combustion chamber shape; rate of combustion chamber heat rejection and/or fuel injection spray pattern, pressure, timing and/or flow rate may be used to positively affect the control of one or more emissions. However, such changes can often adversely affect one or more other emissions possibly causing the emissions to exceed the acceptable limit. For example, as the brake mean effective pressure (bmep) is desirably increased, a tendency arises for NOx emissions in the engine""s exhaust to increase. This problem is accentuated by the need to achieve other critical engine operating characteristics such as fuel economy, high torque output, low operating costs and/or reduced maintenance. As one example, the amount of soot that is entrained in the engine""s lubrication oil can have a profound effect on the cost of operation and the length of service before a major overhaul is required. Soot is very abrasive and can cause high wear if allowed to become entrained in the engine""s lubrication oil to any substantial degree. The amount of soot entrained in the engine""s lubrication oil can be affected by a number of factors such as combustion chamber shape and fuel injection spray angle but changes in these variables can have the undesired effect of actually increasing emissions entrained in the engine""s lubrication oil.
Many attempts have been made to produce an ideal flow pattern for the charge air and fuel within the combustion chamber of an internal combustion chamber. For example, provision of a combustion bowl in the upper region of a piston to cause, among other things, fuel/charge air mixture within a direct injection engine is well known as disclosed the article entitled xe2x80x9cFuture Developments . . .xe2x80x9d, Automotive Industries, Oct. 15, 1952. While most of the combustion bowl designs disclosed in this article appear to be symmetric about a central axis, the article does not address the critical relationship of the combustion bowl shape and the fuel injection path, nor other combustion chamber features, on the specific problems addressed by the subject invention.
A variety of piston designs have been disclosed including symmetrical bowl shaped recesses formed in the upper surface of the piston crown to achieve desired flow patterns within the combustion chamber formed in part by the piston. These bowl configurations are often referred to as xe2x80x9cMexican-hatxe2x80x9d designs. For example, U.S. Pat. No. 4,377,967 discloses an articulated piston assembly including a crown containing a symmetrical combustion bowl in the top surface defined by a cone shaped central floor section which connects at its base to an arcuate surface of revolution coaxial with the central axis of the cone surface wherein the surface of revolution flares upwardly to join with the uppermost surface of the piston. The base of the cone shaped central floor section extends over no more than approximately 50% of the diameter of the bowl. Other similar piston designs are disclosed in U.K. Patent Application No. 2,075,147; and U.S. Pat. Nos. 1,865,841; 3,508,531; 4,242,948 and 5,029,563. However, none of these references disclose any critical size ranges or ratios for the disclosed combustion bowl and chamber designs, suggest the importance of the angle of the fuel spray from the spray orifices in relationship to the combustion bowl shape and specific distances between the piston and both the cylinder head and spray orifices. Thus, these patents fail to disclose that the combustion chamber and piston bowl have crucial dimensions and dimensional relationships that are required to achieve specific engine functionalities including low emissions.
U.S. Pat. No. 5,868,112, assigned to the assignee of the present invention, discloses a piston having a crown containing a combustion bowl shaped to complement the injection fuel spray plume in a manner to maintain very low entrainment of soot in the lubrication oil of the engine and to maintain other engine emissions within acceptable ranges. However, this patent does not appreciate the specific combination of features and dimensions necessary to produce both NOx and particulates below the new regulated limits.
Despite the many examples of combustion chamber arrangements, including piston designs, contained in the prior art, the prior art does not appear to suggest an arrangement that creates the appropriate cooperation between the piston and an injector spray plume to minimize NOx emissions while effectively promoting the oxidation of particulates during combustion by controlling and directing combustion gases in a manner to achieve acceptably low exhaust emissions relative to the new regulated limits. A need, thus, exists for an engine and combustion chamber arrangement that is capable of achieving this combination of functionality.
It is, therefore, one object of the present invention to overcome the deficiencies of the prior art and to provide an internal combustion engine containing a combustion chamber arrangement designed to reduce undesirable engine emissions sufficiently to meet new regulated limits.
Another object of the invention is to provide a combustion chamber arrangement which reduces undesirable engine emissions sufficiently to meet new regulated limits while also minimizing soot in the engine lubrication oil and maintaining other engine performance requirements, such as fuel economy, at acceptable levels.
Another object of the present invention is to provide an engine wherein the shape, position and dimensions of various features of the combustion chamber arrangement, including the piston bowl and the injection spray angle, cause the spray plume to impinge upon and contact the piston bowl surface soon, and much sooner than conventional arrangements, after exiting the injector orifices.
Still another object of the present invention is to provide a diesel engine capable of meeting the new NOx and particulate emission regulations while maintaining acceptable fuel consumption and lube oil soot contamination.
Another object of the present invention is to provide a diesel engine capable of operating below 2.5 grams/b-hp-hr of NOx emissions plus unburned hydrocarbons and below 0.1 grams/b-hp-hr of particulates while also satisfying mechanical design constraints for a commercially acceptable engine.
A more specific object of the subject invention is to provide an engine including a combustion chamber arrangement having dimensions and dimensional relationships to minimize the amount of fuel exposed to oxygen in the chamber during the initial portion of the injection to minimize NOx emissions while ensuring oxidation of sufficient particulates during combustion to minimize both particulates available for entrainment in the engine""s lubrication oil and particulates available for discharge to the exhaust system.
A still more specific object of the subject invention is to provide a key combination of combustion chamber design parameters that together result in a combustion recipe that produces lower NOx emissions than conventional engines.
According to the invention, the above objects and other more detailed objects may be achieved by providing an engine with a combustion chamber arrangement having certain predetermined combinations of combustion chamber design parameters, including specific combustion chamber dimensions and dimensional relationships. For example, in the preferred embodiment, the engine includes an engine body including an engine cylinder, a cylinder head forming an inner face of the combustion chamber and at least one intake port formed in the cylinder head for directing intake air into the combustion chamber. The intake air undergoes a swirling effect during operation to form a swirl ratio in the range of 0.5-2.5. The engine also includes a piston positioned for reciprocal movement in the engine cylinder between a bottom dead center position and a top dead center position. The piston includes a piston crown including a top face facing the combustion chamber. The piston crown contains a piston bowl formed by an outwardly opening cavity while the piston bowl includes a projecting portion having a distal end and an inner bowl floor section extending inwardly at an inner bowl floor angle xcex1 in the range of 16-40 degrees from a plane perpendicular to an axis of reciprocation of the piston. The piston bowl further includes an outwardly flared outer bowl section having a concave curvilinear shape and cross section. The engine further includes an injector mounted on the engine body adjacent the projecting portion of the piston bowl to inject fuel into the combustion chamber. The injector includes a plurality of orifices arranged to form a spray plume. Each of the plurality of orifices includes a central axis oriented at a spray angle xcex1 from a plane perpendicular to the axis of the reciprocation of the piston sufficient to cause the spray plume to impinge on the inner bowl floor section. The spray angle xcex2 is equal to a value causing the spray angle xcex2 minus the inner bowl floor angle xcex1 to be in the range of 0-xc2x7degrees. Each of the plurality of orifices includes an outlet opening having a center wherein the center is a distance L1 from the distal end of the projecting portion and is in the range of 0.5-4 mm.
The plurality of orifices are preferably no more than six orifices and even more preferably less than six orifices. Also, the inner bowl floor angle xcex1 is preferably in the range of 18-30 degrees while the spray angle xcex2 equals a value causing the spray angle xcex2 minus the inner bowl floor angle xcex1 to be in the range of 0-13 degrees. Preferably the swirl ratio is in the range of 0.7-1.5. The engine may also include a cylinder head forming an inner face of the combustion chamber wherein a distance L2 between the center of the outlet opening and the inner face of the cylinder head is in the range of xe2x88x920.5-3 mm. The combustion chamber may be designed such that a distance BH between the top face of the piston crown and the center of the outlet opening is in the range of 0.5-8 mm. Also, the concave curvilinear shape of the outwardly flared outer bowl section may include a radius of curvature R1 in the range of 8-20 mm. The engine cylinder may include a cylinder diameter CD and the piston bowl may include a bowl diameter BD wherein a BD/CD ratio is in the range of 0.5-0.9. The outwardly flared outer bowl section may intersect the top face at an edge having a radius of curvature R2 equal to or less than 1.5 mm. Preferably, the distance L1 is in the range of 1.5-3 mm.
Various other specific combinations of the combustion chamber design parameters described above are also taught by the present invention to achieve the objectives stated herein. For example, an engine having a swirl ratio in the range of 0.5-2.5, an inner bowl floor angle xcex1 in the range of 16-40 degrees, a radius of curvature R1 in the range of 8-20 mm, the spray angle xcex2 equal to a value causing the spray angle xcex2 minus the inner bowl floor angle xcex1 to be in the range of 0-19 degrees, and the distance L2 in the range of xe2x88x920.5-3 mm is also within the teaching of the present invention. Likewise, the specific combinations of swirl ratio, spray angle, inner bowl floor angle, radius of curvature R1 and radius of curvature R2, also form a combustion chamber in accordance with the present invention. In addition, the present invention is directed to an engine body containing the specific combination of design parameters including swirl ratio, inner bowl floor angle, spray angle and a plurality of injection orifices less than six. One or more of the objects of the present invention are also achieved by the particular combination of the swirl ratio taught herein, the inner bowl floor angle, the spray angle, the distance L2 and the distance BH. Furthermore, one or more objectives of the present invention also achieve by the combination of the inner bowl floor angle, the spray angle, the distance L1, the distance L2 and the distance BH. Furthermore, one or more objects of the present invention are also achieved by providing an engine with a combustion chamber having the following combination as taught herein: inner bowl floor angle, spray angle, distance L1, distance L2, and R1. Also, the present invention achieves one or more objects by providing an engine having the combination of design parameters as taught herein including the inner bowl floor angle, the spray angle, the radius of curvature R2, the distance L1 and the distance BH. Of course, other specific combinations of the design parameters taught herein are also deemed to be within the scope of the present invention.